This application relates to and claims priority to corresponding German Patent Application No. 100 53 899.1 filed on Oct. 31, 2000.
1. Field of the Invention
The invention relates to a mounting device for an optical element in an assembly. More specifically the invention relates to a micro lithographic projection exposure objective having a device for mounting an optical element.
2. Description of the Related Art
Mounting techniques are known for rotationally symmetrical optical elements, in the case of which the optical element is connected to a mounting member via a number of elastic elements. Elastic elements are permanently connected, for example by bonding, to the optical element in the region of the outer edge of the latter, and distributed uniformly over the circumference. The elastic elements are mostly designed as spring arms with an axially and a radially situated bent part. In the case of movements of the base points of the spring arms relative to the optical element, such as occur, for example, during lens mounting as a consequence of manufacturing tolerances and inaccuracies in mounting, the optical element essentially experiences via the spring arm an axial force, a radial force and a tangential moment. Depending on the type of base point movement, the three loads resulting therefrom have a different ratio to one another in terms of sign and absolute value. Apart from the type of base point movement, the spring arm geometry and the selection of material are decisive for the ratio between the three loads. If the spring arms are produced from a linearly elastic material, and if the changes in shape of the spring arms are small, for a given load instance the ratio between the three loads on the optical element is independent of the amplitude of the base point movement.
In order to avoid deformations of the optical element because of manufacturing tolerances and mounting inaccuracies, it is known, for example from EP 0230277 B1 and EP 0053463 B1, to create a constructive type of physical mounting for the optical element by means of which the joint between the optical element and the mount is configured in an appropriately soft fashion so that loads occurring can be decoupled as far as possible from the optical element. However, in this case there is a problem that it is impossible to make the joint too soft for example because of the natural frequency requirements.
Reference may be made further to DE 198 25 716 A1 and U.S. Pat. No. 5,428,482 regarding the prior art.
It is the object of the present invention to create a device for mounting an optical element, the aim being that loads occurring do not have a negative effect on the optical element, in particular not in the form of surface deformations.
According to the invention, this object is achieved by means of a mounting device for an optical element in an assembly having the following features:
1.1 a support member and two or more gear elements,
1.2 the support member is connected to an external mounting structure and, via base points, to the gear elements,
1.3 via top points, the gear elements are connected to the optical element directly, or are connected to the optical element indirectly via a mount arranged therebetween,
1.4 the top points of the gear elements are located in planes of symmetry of the optical element which are defined by an axial axis and a radial axis of the optical element,
1.5 the gear elements are arranged and dimensioned such that in the event of disturbing influences a compensation effect is produced with regard to a deformation of an optical surface of the optical element,
1.6 the compensation effect being provided when a RMS value of the sum of those partial deformations of the optical surface of the optical element which are caused by an axial force, a radial force and a tangential element at an arbitrary top point of a gear element is less than or at least equal to one half of the largest RMS value of one of the three said partial deformations.
A ratio in which the two-wave components of deformation of the optical surface of the optical element cancel one another out is found by a configuration, according to the invention, of the gear elements and by an appropriate selection of the joining points of the optical element or of an optical assembly in the case of one or more of the three load instances, that is to say an axial base point displacement, a radial base point displacement and a tangential base point rotation for the axial forces, radial forces and the tangential moments at each joining point. Assuming that the total stiffnesses remain the same in all design variants of the gear elements, in the case of complete compensation of the two-wave deformation components a point is reached at which the aspherical component of the deformation is close to a minimum as a function of the load ratios. The minimum of the aspherical deformation can then be found easily by means of small corrections in the load ratio provided by the gear elements.
Mounting carried out in this way according to the invention uses the transmission of deformations of the base point plane, which in the case of lenses is mostly a mounting ring, to the optical element by comparison with mounting with the aid of a spring arm mount of known type with similarly large stiffnesses by one to two orders of magnitude.
Moreover, it is possible by means of the type of mounting described to minimize deformations which are caused by bearing reactions which are in equilibrium with accelerating forces such as, for example, the weight force. In other words, this means that acceleration-induced deformations of the optical surface are likewise minimized.
According to the invention, the compensation mechanism requires that the two-wave partial deformations which are caused by the axial forces, radial forces and the tangential moments at the individual joining points, have the same angular orientation. For this purpose, all the gear elements must lie in a plane of symmetry of the optical element. This is so at any point in the case of lenses, and in the case of optical elements whose outer contour is not found, a sufficient number of planes of symmetry must be present for mounting.
Analyses carried out on models, such as a finite element analysis, for example, of the optical element, or with the aid of suitable measurements on a real optical element, can be used to determine for each joining point the way in which axial and radial forces and tangential moments introduced there affect the optical surface. Whilst, for example, a Zernike analysis can supply the sensitivity of the two-wave deformation of the optical surface to the respectively introduced load. The sensitivities thus determined can be used to determine for each joining point one or more ratios of the three loads to one another in the case of which the desired compensation effect occurs.
The solution according to the invention is therefore joining elements or gear elements which provide for one or more types of base point movements a suitable ratio for the compensation of the two-wave deformation components, and can be designed as a mechanical spring joint gear. The optical element, for example a lens, can be connected either directly to the gear elements or indirectly via a mount arranged therebetween.
According to the invention, it is not the forces acting on the optical element or the mount which are compensated, but the partial deformations on the optical surface which are caused by these forces. Although as the forces at the joining points to the optical element or to the mounted optical element are defined, specifically as axial force, radial force etc., they cannot cancel one another out, since they are orthogonal, each of the six individual loads causes a partial deformation on the optical surface, depending on the joining point. These partial deformations are no longer orthogonal, but partially rendered highly unidirectional. It is precisely this unidirectional characteristic which provides the basis for the capability for compensation.
In order to achieve exactly reproducible results, the gear elements should have struts, articulated joints leaf springs or combinations of said elements, the aim being for the articulated joints to be free from backlash and latching moments. Solid articulated joints can preferably be used for this purpose.
The compensation effect is provided according to the invention for disturbances which lead to a variation in the relative position and the orientation of the base points of the gear elements. The same also holds for variations in the joining loads at the top points of the gear elements which are caused by accelerations.
Particularly advantageous-results are achieved when the diameter/thickness ratio of the optical element is greater than 2:1.
In the case of an optical element provided with a mount, the joints of the optical element to the mount should advantageously be in the range between the outside diameter and half the outside diameter of the optical element.
The deformation distributions caused respectively by axial forces, radial forces and tangential moments (individual loads) must be very similar with regard to their characteristic spatial distribution, because this is a precondition for being able to achieve a compensation effect by means of suitable gear elements.
The further outward the joining point is situated, the more similar become the deformation distributions of the respective individual loads of a joining point, and the more effectively a compensation effect can be achieved. If the joining point moves very far inward, the deformation distributions of the individual loads are dissimilar starting from a specific limit, and so effective compensation is no longer possible.